1. Field of the Invention
This invention relates to a textile fabric and yarn composed of synthetic fibers coated with fluoropolymer and also to the preparation of textile sheet material and yarn and the use thereof.
2. Description of Related Art
EP 0 327 047 B1 discloses aqueous fluoropolymer formulations containing in finely divided form from 25% to 60% by weight of a fluoropolymer, from 1% to 5% by weight of an organic compound containing at least two isocyanate groups and optionally up to 10% by weight of auxiliary or additive materials. These fluoropolymer formulations are used for coating yarn and textile sheet materials composed of dyed or undyed, flat or textured synthetic filaments or fibers and for bonding sewing yarn. The textile sheet materials coated with such fluoropolymer compositions comprise, at least in the fluoropolymer layer which is immediately adjacent to the fiber surface, one or more adhesion-promoting constituents which are derived from an organic compound containing two or more isocyanate groups. The adhesive strength of the fluoropolymer layer as measured in line with DIN 53530 is at least 10 daN/5 cm.
EP 0 224 262 A discloses laminating sheetlike textile materials on both sides with a polyvinyl fluoride film. In this known process, a solution of the polymer in an organic solvent is poured onto a conveyor belt to form a thin layer which is caused to gel by heating. The gel layer is then pressed onto the surface of the textile material to be laminated. In this process, the fluoropolymer is only fixed on the surface of the textile material and there is virtually no impregnation of the individual filaments with the fluoropolymer.
German Offenlegungsschrift DE 33 01 270 A discloses a process for sheathing fiber or filament yarns with a fluorine-containing polymer. In this costly and inconvenient process, the filament to be sheathed is pulled through a central hole in an annular spinneret while at the same time a tube of fluoropolymer is extruded from the annular spinneret. The extruded tube therefore forms a loose-fitting sheath around the filament drawn out of the central hole. No firm bond is established between the fluoropolymer tube and the filament sheathed therewith.
It is also known to manufacture awnings, air-houses, flexible containers and similar products by coating textile sheet structures, usually woven fabrics, preferably those made of synthetic organic fibers or filaments, with polymer materials, usually with polyvinyl chloride (PVC). This coating is effected by impregnating the textile materials in suspensions of polyvinyl chloride in organic liquids. In the course of this coating, even the individual filaments of the textile material become enveloped by the polyvinyl chloride coating. To obtain sufficient adhesion between the polyvinyl chloride coating and the synthetic fiber, the coating is carried out in two stages. First a basecoat is applied comprising a mixture of a PVC paste or suspension with an adhesion promoter; this is followed by the application of a topcoat comprising a straight PVC formulation. The adhesion promoters which are suitable for this process are known. It is usual to use two-component adhesives comprising an organic substance having a plurality of hydroxyl groups, preferably a hydroxyl-containing polyester, and an organic substance having a plurality of isocyanate groups.
It is also already known to coat materials such as threads or sheetlike structures made of organic synthetic fibers with fluoropolymers in order that particularly advantageous properties, for example a low coefficient of friction, a high chemical resistance and a soil-repellent effect, may be conferred on their surfaces. Toward this end, the synthetic fiber materials are impregnated or slip-coated with commercially available aqueous dispersions of fluoropolymers and the resulting polymer coating is fixed by means of a heat treatment.
U.S. Pat. No. 3,071,565 discloses a process for converting linear elastomeric or thermoplastic chlorofluoropolymers such as, for example, mono- or copolymers of 2-chloro-perfluoropropylene, chlorotrifluoroethylene, bromotrifluoroethylene, trifluoroethylene, chlorofluoroethylene and vinylidene fluoride, into cross-linked, space polymers in order that their solubility and their thermoplastic flow may be reduced and the elastomers may be subjected to a mild vulcanization as it were.
This object is said to be achieved by this reference by allowing the chlorofluoropolymers to react with polyisocyanates in the presence of moisture. However, the reference says nothing about coating fiber materials with fluoropolymers and does not in any way address the problems of adhesion between fluoropolymers and synthetic fibers.
However, in order to make composite materials based on fluoropolymer-treated synthetic fibers suitable for a wide range of applications, for example the manufacture of membranes for textile building construction, flexible containers, conveyor belts and fabric tubes, it is absolutely necessary that the fluoropolymer should possess adequate adhesion to the synthetic fiber. Adhesion or adhesive strength is here to be understood as meaning the resistance to separation of base material and coating for a 5 cm wide strip as determined in line with German standard specification DIN 53530. Adequate performance capability of the composite is ensured when, depending on the planned application, adhesion values from 100 to 150 N/5 cm are achieved. There are some applications where it is even desirable to have adhesion values of more than 200 N/5 cm.
However, the production of strongly adherent fluoropolymer coatings on synthetic fiber materials presents even greater difficulties than the production of polyvinyl chloride coatings. This is because it is found that fluoropolymers are far more inert with regard to synthetic fibers, for example polyester fibers, polyamide fibers or aramid fibers, than is polyvinyl chloride; that is, they show great reluctance to enter permanent physical or chemical bonds with synthetic fiber surfaces. Moreover, fluoropolymers which, on the basis of their physical data, might be thought suitable for use as a coating agent for synthetic fibers are generally commercially available in the form of aqueous dispersions or pastes. It is therefore not possible using these known fluoropolymer dispersions or pastes to produce coatings on synthetic fiber material which exhibit adequate adhesive strength for all the industrial uses mentioned above.
Nor is it possible to obtain a significant improvement in the adhesion of fluoropolymer coatings by using the one- or two-component adhesion promoters used successfully in the production of PVC coatings.
The firmly adherent fluoropolymer coatings on synthetic fiber which are known from EP 0 327 047 B1 contain an organic compound having a plurality of isocyanate groups instead of a conventional adhesion promoter. Known fluoropolymers include commercially available tetrafluoroethylene copolymers which contain hexafluoropropylene and vinylidene fluoride structural repeat units.
Organic compounds which have a plurality of isocyanate groups and which are incorporated in fluoropolymer formulations are available as a commercial material. Useful di- and polyisocyanates include for example the isomeric 2,4-diisocyanatotoluene and its mixtures, 1,5-diisocyanatonaphthalene, diisocyanatodiphenylmethane and its technical grade isomer mixtures, dimerized and trimerized 2,4-diisocyanatotoluene, adducts of diisocyanatotoluene with trimethylolpropane and tris[isocyanatohexyl]biuret. Particular preference for use in the fluoropolymer formulations according to the invention is given to the aforementioned derivatives of diisocyanatotoluene, especially its dimerization product, which has heretofore been marketed by Bayer AG under the name of ®Desmodur TT.
Gas phase fluorination provides prolonged high-level activation to polymeric surfaces. Fluorine is the most reactive element in the Periodic Table and therefore can be made to react with almost all organic and inorganic compounds in controllable reactions even at room temperature without further activation by means of catalysts or UV light. Fluorination replaces some of the hydrogen atoms in the polymer surface by fluorine atoms. This creates an active surface on which mechanical and chemical bonds can form. Gas phase fluorination is available from various suppliers on a tolling basis and forms part of the background art. A gas phase pretreatment with fluorine in the absence of oxygen, ie in a vacuum or in a fluorine/inert gas mixture, is a fluorination in the proper sense. When, in contrast, oxygen is likewise present in the reaction space, the reaction is an oxyfluorination. With this kind of activation, the free-radical sites on the carbon chain are observed to locate not only fluorine atoms but also hydroxyl and carboxyl groups, which likewise enhances surface activity. Suitable substrates for oxyfluorination include all industrial textiles, films, foams and the like, to render them hydrophilic for example. Fluorination and oxyfluorination each employ a mixture of up to 10% of fluorine in 90% of inert carrier gas or oxygen-enriched carrier gas (eg air), whereby industrial textiles, polymeric films, foams, yarns and the like are generally activated by the inline process in which the material passes from reel to reel through a chamber of the reactive medium. The actual condition chosen in each case are dependent inter alia on the treatment duration, the polymer type, the yarn properties (filament fineness, degree of entangling, surface pretreatment, etc.) and the fabric construction (density, weight, etc.). (Compare, for example, Dr. R. Milker, Neuwied, A. Koch, Lauterbach “Oberflächenfluorierung von Textilien zur Erhöhung der Haftfestigkeit”, Chemiefasern/Textilindustrie (Industrie Textilien), Volume 39/91, July/August 1989).